The nucleus, like all cellular organelles, is composed of a unique set of proteins responsible for its structure and function. The long term objective of the proposed research is to develop a detailed understanding of the mechanisms that govern the movement of proteins and RNAs in and out of the nucleus. The problem is being studied both biochemically and genetically in the yeast, Saccharomyces cerevisiae. Proteins important for nuclear assembly and export of RNA have been identified by isolation of mutants. An in vitro nuclear protein assembly reaction has been developed and the roles in transport of proteins in the cytosol, the nucleus, and the nuclear pore complex can now be defined. In this proposal, experiments designed to further investigated the function of these proteins and identify other components important for movement of macromolecules across the nuclear envelope are described. The specific aims are; 1) to test the hypothesis that Np13 is an active carrier of mRNA from the nucleus and is involved in nucleocytoplasmic exchange, 2) to understand the role of a GTP- binding proteins, Gsp1, in nuclear transport and cell growth, 3) to complete the characterization of mutants that affect nuclear protein localization, and 4) to further define the requirements for nuclear transport using a well-defined in vitro reaction. The detailed picture of how proteins and RNAs move in and out of the nucleus that should emerge from these experiments will aid our understanding of cell cycle regulation as well as viral infection and assembly. The ability to target proteins to the nucleus or to selectively disrupt nuclear import of certain proteins may have potential therapeutic value.